A solid laser apparatus is known comprising a semiconductor laser for generating a laser beam, a nonlinear optical device for receiving the laser beam from the semiconductor laser and emitting its harmonic light, an optical detecting means used as a monitor for detecting the intensity of the light emitted from the nonlinear optical device, and an output control circuit for driving the semiconductor laser so as to maintain the intensity of the light at a predetermined level (See Citation 1).
Another solid laser apparatus is known having a microchip laser crystal designed for being excited by the laser beam emitted from a semiconductor laser, provided with its crystal end faces coated for use as an optical resonator, and located at the upstream of an nonlinear optical device (See Citation 2).
Citation 1 disclosed in Japanese Patent Laid-open Publication (Heisei)7-106682.
Citation 2 disclosed in Japanese Patent Laid-open Publication (Heisei)4-503429.
(Problems that the Invention is to Solve)
FIG. 20 illustrates an actual measurement profile of gain transmission characteristic and phase transmission characteristic of the nonlinear optical device and the microchip laser crystal in such a conventional solid laser apparatus.
As shown in FIG. 20, the gain peak of a signal appears about 11 MHz. The frequency at the gain peak is referred to as a relaxation oscillating frequency fk of the solid laser apparatus. Also, the profile of FIG. 20 illustrates the phase of the signal is inverted around the relaxation oscillating frequency fk and retarded by substantially 90 degrees in the vicinity of the relaxation oscillating frequency fk.
FIG. 21 illustrates a measurement profile of optical noise waveforms in the solid laser apparatus.
The profile of FIG. 21 illustrates the oscillating frequency of optical noise at substantially 11 MHz which is equal to the relaxation oscillating frequency fk shown in FIG. 20.
The solid laser apparatus having the phase transmission characteristic shown in FIG. 20 allows a range of frequencies lower than the relaxation oscillating frequency fk to be successfully controlled by the negative feedback controlling action of the output control circuit.
However, if a range of frequencies higher than the relaxation oscillating frequency fk contains any external interruption such as a circuitry noise due to the inversion of the phase transmission characteristic in the vicinity of the relaxation oscillating frequency fk as shown in FIG. 20, it may cause the control system to start oscillation and disturb the negative feedback controlling action thus failing to effectively attenuate the optical noise. Also, if an external interruption such as the circuitry noise is contained in the range of frequencies lower than the relaxation oscillating frequency fk, it may be treated as an optical noise thus allowing the attenuation of optical noise with much difficulty. Moreover, if the optical noise in the vicinity of the relaxation oscillating frequency fk is delayed by substantially 90 degrees in the phase, it may be attenuated only to a limited level by the negative feedback controlling action of the output control circuit.
On the other hand, FIG. 22 illustrates a profile of gain transmission characteristic of the nonlinear optical device and the microchip laser crystal in such a conventional solid laser apparatus.
As shown in FIG. 22, the relaxation oscillating frequency fk of a solid laser apparatus appears about 12 MHz.
FIG. 23 illustrates a profile of phase transmission characteristic of the nonlinear optical device and the microchip laser crystal in the conventional solid laser apparatus.
As shown in FIG. 23, the phase of the signal is inverted in the vicinity of the relaxation oscillating frequency fk.
The solid laser apparatus having the phase transmission characteristic shown in FIG. 23 allows any interruption (low frequency noise) in the output at frequencies lower than the relaxation oscillating frequency fk to be successfully controlled by the negative feedback controlling action of the output control circuit. This is because the common operating frequencies of the solid laser apparatus are sufficiently lower than the relaxation oscillating frequency fk.
However, since the phase transmission characteristic is inverted at the relaxation oscillating frequency fk as shown in FIG. 23, the negative feedback action will be turned to a positive feedback action hence failing to suppress an interruption (high frequency noise) in the output at the frequencies higher than the relaxation oscillating frequency fk.
It is hence an object of the present invention to provide a solid laser apparatus which can effectively attenuate optical noises.